Optical tweezers use a highly focused beam of light to provide an attractive or repulsive force to physically hold and move microscopic dielectric objects. Typically, this beam of light is focused by sending it through a microscope objective or other short focal length lens. The narrowest point of the focused beam of light, known as the beam waist, contains a very strong electric field gradient. Dielectric particles are attracted along the gradient to the region of strongest electric field which is the center of the beam of light. The beam of light also tends to apply a force on particles in the beam along the direction of beam propagation. This is known as the scattering force and results in the particle being displaced slightly downstream from the exact position of the beam waist.
Unfortunately, optical tweezers are limited to the small volume of light in the focal region and therefore are unsuitable for large areas of volumetrically dispersed objects. Additionally, optical tweezers do not work well across large depths. Further, the components needed to make optical tweezers are expensive.